1. Field of the Invention
The present invention relates to a positive resist composition which can be used in the step of producing semiconductors such as ICs, producing circuit substrates for liquid crystal or thermal head, or in other photo fabrication steps, and to a pattern-forming method using the resist composition. In particular, it relates to a positive resist composition suitable for exposure with an immersion exposure type projection exposure apparatus employing deep ultraviolet rays having a wavelength of 300 nm or shorter as an exposure light, and to a pattern-forming method using the composition.
2. Description of the Related Art
With the trend toward size reduction in semiconductor elements, the wavelengths of exposure lights are decreasing and the numerical apertures (NA) of projection lenses are increasing. An exposure apparatus which has an NA of 0.84 and employs an ArF excimer laser having a wavelength of 193 nm as a light source has been developed so far. As is generally well known, resolution and focal depth can be expressed by the following equations:(Resolution)=κ1·(λ/NA)(Focal depth)=±κ2·λ/NA2 wherein λ is the wavelength of the exposure light, NA is the numerical aperture of the projection lens, and κ1 and κ2 are coefficients relating to the process.
An exposure apparatus employing an F2 excimer laser having a wavelength of 157 nm as a light source is being investigated for the purpose of enhancing resolution by using a shorter wavelength. However, use of this apparatus is disadvantageous in that materials for the lens to be used in the exposure apparatus and materials for resists are considerably limited due to the use of such a shorter wavelength. Because of this, the cost of apparatus and material production is high and it is extremely difficult to stabilize quality. There is hence a possibility that an exposure apparatus and a resist which have sufficient performances and stability might be unavailable in a desired period.
The so-called immersion method has been known as a technique for enhancing resolution with respect to optical microscopes. In this method, the space between the projection lens and the sample is filled with a liquid having a high refractive index (hereinafter also referred to as “immersion liquid”).
This “immersion” has the following effects. In the immersion, the resolution and the focal depth can be expressed by the following equations on the assumption that NA0=sin θ:(Resolution)=κ1·(λ0/n)/NA0 (Focal depth)=±κ2·(λ0/n)/NA02 wherein λ0 is the wavelength of the exposure light in air, n is the refractive index of the immersion liquid relative to that of air, and θ is the convergence half angle of the light.
Namely, the immersion produces the same effect as the use of an exposure light having a wavelength reduced to 1/n. In other words, in the case of an optical projection system having the same NA, the focal depth can be increased to n times by the immersion.
This is effective in all pattern shapes and can be used in combination with a super resolution technique such as the phase shift method or the deformation illumination method.
Examples of apparatus in which this effect is applied to the transfer of fine circuit patterns for semiconductor elements are introduced in JP-A-57-153433, JP-A-7-220990, etc.
Recent progresses in the immersion exposure technique are reported in SPIE Proc 4688, 11 (2002), J. Vac. Sci. Technol. B 17 (1999), SPIE Proc 3999, 2 (2000), and WO2004-077158.
In the case where an ArF excimer laser is used as a light source, pure water (refractive index at 193 nm: 1.44) is thought to be most promising from the standpoints of safety in handling and transmittance and refractive index at 193 nm. Although solutions containing fluorine are being investigated for use in the case of using an F2 excimer laser as a light source from the standpoint of a balance between transmittance and refractive index at 157 nm, no immersion liquid has been found which is sufficient from the standpoints of environmental safety and refractive index. In view of the degree of the effect of the immersion and the degree of completion of resists, the technique of immersion exposure is thought to be employed first in an ArF exposure apparatus.
Since the advent of resists for KrF excimer lasers (248 nm), the technique of image formation called chemical amplification has been used as a resist image formation method for the purpose of compensating for a sensitivity decrease caused by light absorption. To describe the positive chemical amplification type image-forming method as an example, it is a method for forming an image which comprises exposing a resist film to light to thereby cause an acid generator in the exposed areas to decompose and generate an acid, subjecting the resist film to post-exposure bake (PEB) to utilize the resultant acid as a reaction catalyst to convert alkali-insoluble groups into alkali-soluble groups, and removing the exposed areas by alkali development.
Although resists for an ArF excimer laser (wavelength: 193 nm) using this chemical amplification mechanism are predominating at present, they involve the problem that a formed line pattern can collapse to cause defects upon production of a device. Thus, this problem has been required to solve.
It has been pointed out that, when a chemical amplification type resist is subjected to immersion exposure, the resist layer might be deteriorated or ingredients exerting detrimental influences might ooze from the resist layer into the immersion liquid, since the resist layer comes into contact with the immersion liquid upon exposure. WO2004-068242 describes a case wherein the resist performance is changed when the resist for ArF exposure is dipped in water before and after exposure, and this is indicated as a problem in the immersion exposure.
Also, in conducting immersion exposure, there can result a circular defect called water mark, and this defect is required to remove.